1. Field of the Invention
This invention relates to a material for producing a polyurethane (hereinafter referred to as polyurethane material) used for reactive injection molding (hereinafter referred to as RIM) of a cover of a pad to be set on an automobile stearing wheel.
2. Description of the Related Art
The term "flon" used in the specification of the invention is a generic name for fluorocarbons and chlorofluorocarbons.
The conventional pad covers have been made by RIM using as foaming agent a polyurethane material blended with a specific flon CFC-11 (trichloromonofluoromethane) or an alternate flon such as HCFC-123 (2,2-dichloro-1,1,1-trifluoroethane), HCFC-141b (1,1-dichloro-1-fluoroethane) or HCFC-22 (monochlorodifluoroethane). This molding method is explained briefly below.
A polyol mixture comprising a premix of a polyol, a flon, a catalyst and a pigment and an isocyanate are mixed in a specified ratio by means of impingement mixing in a mixing head to prepare a polyurethane material, which is cast into the cavity of a mold. An urethane reaction is immediately induced between the polyurethane mixture and the isocyanate through the medium of the catalyst, and the flon is volatilized by the heat of reaction to form a foam. Thus, the polyurethane material is gradually foamed and caused to flow in the cavity. By this process, a densely foamed core portion is formed in the inside of the molded cover while a sparsely foamed compact skin is formed at the surface portion of the cover due to the cooling action of the mold. Thus, the molded pad cover has an integral-skin foam structure.
Recently, the detrimental effect of flon on the ozone layer in the atmosphere has become a matter of serious concern, which has prompted many of the flon-producing countries to regulate the production of flon, and it is expect that use of not only the specific flon CFC-11 but also the alternate flons, such as HCFC-123, HCFC-141b and HCFC-22, will be totally banned in the near future. In view of the above, the present inventors have successfully demonstrated the feasibility of RIM of a non-foamed or sparsely foamed pad cover by using a polyurethane material containing no flon as described below.
First, by using a polyurethane material prepared without adding any flon in otherwise the same way as in the prior art (refer to Comparative Example 1), a cover 51 (FIG. 3) of an air bag-housing pad with a substantial volume was molded. The fluidity of the polyurethane material was poor however, from the time immediately after casting into the mold, and a lacuna 53 was produced at an end (the remotest end from the material casting gate 52) of the molding. It is thought that the catalyst promoted the urethane reaction, causing rapid thickening. The absence of flon, which is known to reduce viscosity of the polyurethane material and enhance foaming, led to the unacceptable results.
A number of strategies have been directed at enhancing the fluidity of the polyurethane material, such as reducing the content of the catalyst (Comparative Examples 2 and 3), lowering the viscosity of the material by selecting the optimal composition of the polyol mixture for producing a similar viscosity lowering action as flon (Comparative Example 4), and conducting air loading at a high rate for producing an effect similar to the foaming action of flon (Comparative Examples 5 and 6), each of these strategies however, have been unsatisfactory to enhance the fluidity of the polyurethane material. Lacuna was still formed in the molding.
The object of the present invention is to eliminate the above-described difficulties and to provide a novel polyurethane material which, although containing no foaming agent such as flon, is capable of molding a non-foamed or only sparsely foamed pad cover free of lacuna.